Gels comprising alumina combined with various organic substances are known in the prior art. However, such gels generally take the form of a white floc rather than a stable clear gel.
For example, Montgomery et al U.S. Pat. No. 3,417,028 claims addition of polyvinyl alcohol, polyethylene glycols, polyethylene oxides, methyl celluloses, and polyacrylamides to solutions containing alumina hydrate. The organic polymers were reacted with the alumina in solutions made basic by addition of ammonium hydroxide. Because the reaction was carried out in basic solution, the products were white flocs rather than clear gels even though the patent refers to such products as "gels" or "hydrous gels".
Andre et al U.S. Pat. No. 3,993,590 describes a process for preparing silica-alumina beads by polymerizing alumina and silica precursors with a water-soluble monomer comprising an acrylic compound. The reaction is carried out in an aqueous mixture preferably having a pH of about 3-4. One of the alumina precursors is an alumina hydrosol made by hydrolyzing an aluminum alcoholate or other water-soluble aluminum compound. A preferred monomer is acrylic acid. The polymerization reaction product comprises beads which are washed, dried, and calcined to destroy organic matter.
Andre et al do not suggest substituting a vinyl alcohol-vinyl acetate copolymer for the acrylic monomers preferred for their polymerization reaction. Addition of an organic plasticizer to the reaction mixture described above would be inconsistent with their ultimate objective of producing calcined alumina-silica beads which are free of any organic matter. Andre et al do not teach or suggest the toughened gel and substantially crack-free film claimed herein.
As used herein, the terms "monolithic gel" and "stable gel" refer to a gelled mass characterized by substantially no separation of a liquid phase. The term "floc" refers to a combination or aggregation of suspended particles in such a way that they form small clumps or tufts. A stable gel made in accordance with the present invention is generally transparent or slightly opalescent whereas an alumina hydrate floc is generally white. The term "gel" is often used in the prior art to describe what is called a floc herein.
The expression "alumina hydrate" refers to Al.sub.2 O.sub.3 .multidot.x H.sub.2 O, wherein x varies from 1 to 3. In other words, the water of the alumina hydrate varies from 15.0 to 34.6 percent by weight of the alumina hydrate, determined by calcination at 538.degree. C. (1000.degree. F.) for one hour.
It is a principal objective of the present invention to provide a method for preparing a stable clear gel comprising alumina hydrate combined with a vinyl alcohol-vinyl acetate copolymer.
A related objective of the invention is to provide a stable monolithic gel comprising alumina hydrate combined with a vinyl alcohol-vinyl acetate copolymer.
A further objective of the invention is to provide a method for transforming the clear gel of the invention into a crack-free, generally transparent film.
Additional objects and advantages of the present invention will become apparent to persons skilled in the art from the following specification and claims.